Orange zest is a way to bring orange flavor to a dish without the bulk and potential bitterness that whole orange peel can bring. The zest is the outer part of the orange peel and can be scraped or shaved from the fruit with the use of a zester, a cheese grater or a sharp knife. The use of zest not only adds the most aromatic part of the orange fruit to your dish, it does so unobtrusively. The shavings or slivers will be relatively fine and will blend into most foods.
There is no one ideal orange zest substitute that will work in all dishes. The one you choose will depend on the dish you are making.
Your best bet: Lemon zest
Lemon zest will provide a citrus sweetness to dishes that require orange zest. Because both come from citrus fruits, their benefits will be similar and the lemon flavor will pair well with the spices that are most commonly used with orange zest. Lemon zest is equally effective in both savory and sweet dishes. You can use it as a replacement for orange zest in orange chicken or in cookies and cakes.
The zests from lemons and oranges are equally pungent, which means that you can use lemon zest as a 1:1 substitute for orange zest.
A decent second choice: Orange peel
Orange peel is the whole exterior of the orange fruit, which means that it includes both the zest and the pith. The fact that it actually has the zest attached is what makes it a great substitute. You can use it fresh or you can dry it to extend its shelf life. Dried orange peel has the benefit of not being quite as bitter and the fresh version.
Orange peel has more bulk than orange zest and can be somewhat bitter since it has the pith attached; however, it will still provide the orange flavor. Orange peel is best added whole so that the essential oils can infuse into the dish. You can remove the peel before serving. This means that it may not be ideal for cakes and cookies but can be used in braised dishes and for sauces like the orange sauce for Chinese orange beef.
The peel of one whole orange should give the same amount of flavor as the zest of a whole orange.
In a pinch: Orange juice or concentrate
The juice of the orange is another source of the flavor that you get from orange zest; however, it is not quite as aromatic. As a result, many recipes that require a strong orange flavor list both the juice and the zest as ingredients. Because it is a liquid, it may not be ideal for all recipes. In cake batters, you may need to replace all of one or more of the liquids with orange juice to get an adequate orange flavor.
Orange concentrate is orange juice with some of the liquid removed. Orange concentrate is a thick and sweet reduction of orange juice. The flavor is generally stronger, as is the sweetness. Because of the high sugar content and the fact that it is a liquid, it is not always an ideal orange zest substitute. It can be effective in orange sauces and similar applications.
The amount of orange juice or concentrate that you use to replace zest will vary strongly depending on the dish you are making. It is sometimes recommended that you use two tablespoons of orange juice or two teaspoons of orange concentrate to replace a single teaspoon of zest.
You can use an orange extract as an orange zest alternative in some recipes. Extracts vary in strength but can be very potent, so use with caution to avoid ruining your dish.